The object of these investigations is to elucidate the mechanisms responsible for the phenomenon of myocardial reactive hyperemia in the dog. At the same time, the possibility that reactive hyperemia responses in the human heart can be used to indicate coronary vasodilator reserve as well as adequacy of coronary bypass surgery will be examined. The contemplated studies will be carried out in a canine model and in the human heart at the time of coronary bypass surgery. In the dog, various physiological, pharmacological and correlative histochemical studies will be carried out. In the human, phasic flow and reactive hyperemia responses will be recorded before and after coronary surgery and correlated with the angiographic and anatomic findings and with patency rates as determined in follow-up arteriographic studies.